An enzyme-linked immunosorbent assay was developed to quantify immunoglobulin (Ig) in the serum and mucus of channel catfish (Ictalurus punctatus). The capture antibody (Ab) was a monoclonal Ab to channel catfish Ig and the detector was a polyclonal goat serum against channel catfish Ig. Detectable levels of channel catfish Ig were 0.1 microgram ml-1 and coefficients of intra- and interassay variations were 20% and 15%, respectively. Serum Ig concentration was found to increase from 0.36 +/- 0.25 to 0.49 +/- 0.26 mg ml-1 between 3 and 15 months of age. In contrast, mucus Ig concentration slightly decreased from 0.55 +/- 0.52 to 0.45 +/- 0.82 ng cm-2 of skin between 3 and 15 months of age. Mucus Ig concentrations were highest immediately caudal to gill covers and between pectoral to anal fins, at concentrations of 0.45 +/- 0.82 and 0.34 +/- 0.67 ng cm-2, respectively. The concentration of mucus Ig between anal and caudal fins and on the ventral skin between gill covers and pectoral fin was 0.18 +/- 0.42 and 0.09 +/- 0.14 ng cm-2, respectively. The kinetics of Ig production following infection with Edwardsiella ictaluri showed that serum Ig concentrations markedly increased (20.92 +/- 32.75 mg ml-1) at 13 days post-infection. The increase in mucus Ig concentrations occurred 14 days later (0.80 +/- 1.91 ng cm-2). Individual variation in both serum and mucus Ig production increased considerably during the course of infection (+/-0.23 to +/-32.75 mg ml-1 for serum Ig and +/-0.1 to +3.41 ng cm-2 for mucus Ig). The results suggest that E. ictaluri infection causes higher Ig production than that found in non-infected fish of the same age. Ab response to E. ictaluri was detected in the serum but not in the mucus of infected fish. Our results show differences in serum and mucus Ig concentrations with age and after infection with E. ictaluri.